1. Field of the Invention
This invention relates to transparent soft, tough contact lenses for correcting vision deficiencies of the human eye.
2. Description of the Prior Art
Contact lenses are of two general types, scleral and corneal. Lenses of the scleral type cover a substantial area of the eye during use and seal off circulation of tears and the atmosphere. This causes partial asphyxiation which affects the metabolism and vision of the eye. The more recently developed corneal lenses, because of their relatively smaller size and lighter weight, cause less irritation to the cornea and have achieved greater acceptance. However, the "break in" period necessary to accustom the cornea of the wearer to a corneal lens usually extends over a considerable period of time. Some persons have found contact lenses to be intolerable because of the eye irritation resulting during and after any extended period of continuous wear.
Contact lenses of both the corneal and scleral types have been commonly constructed of poly(methyl methacrylate) (PMMA). A lens made of such material is hard and stiff and when introduced for the first time on the eye of a person, the sensation is that of a foreign body. A fraction of the people who try PMMA lenses become adapted after a break-in period and can successfully wear the lenses. However, a larger fraction of the individuals who attempt to wear PMMA lenses never become wearers since they cannot adapt to such hard and stiff lenses because of discomfort, irritation, excessive tearing, and the like. Even those individuals who do adapt successfully to PMMA lenses, however, suffer an awareness of the lens in the eye.
Contact lenses have also been made from fluorinated copolymers, as for example, those disclosed in U.S. Pat. No. 3,542,461 issued to Girard et al. These lens are also hard and stiff and suffer from the same disadvantages as the PMMA lenses.
Contact lenses have also been made from other materials such as hydrogels and silicones. These are exemplified by U.S. Pat. No. 3,220,960 to Wichterle et al. and U.S. Pat. No. 3,228,741 to Becker, respectively. Lenses made of these materials are referred to as soft lenses and are an improvement over hard and stiff lenses in being very comfortable, a property which is immediately apparent to the wearer. However, the hydrogel and silicone lenses have serious drawbacks which include poor or variable visual acuity resulting from difficulties in their manufacture and, in the case of the hydrogel lens, variable water absorption, tendency to absorb foreign substances, including tobacco smoke, bacteria, fluorescein, wetting solutions, mascara, and the necessity for daily sterilization. Hydrogel lenses, after a period of prolonged use, may acquire proteinaceoous deposits which can lead to physical discomfort and lessened visual acuity. In addition, the hydrogel and silicon lenses are weak and subject to damage by tearing and breaking. A discussion of the disadvantages of such lenses is found in "Precision-Cosmet Digest," Vol. 5, No. 9, April 1965.